Hydrocarbons

Carbon and hydrogen alone build fuels, plastics and more. Learn the four families and the one idea that separates them.

High schoolIntro OrganicUni Year 1
⏱️ About 18 min

Natural gas, gasoline, candle wax, the ethene that ripens your fruit, and the benzene ring inside countless medicines — all of them are just carbon and hydrogen. Learn how those two elements pack together in four different ways, and a huge slice of organic chemistry falls into place.

💡
The big idea: Hydrocarbons contain only carbon and hydrogen. Carbon forms four bonds, so it chains and rings endlessly. The single dividing line is whether every carbon–carbon bond is a single bond (saturated) or some are double/triple bonds (unsaturated) — and that decides how the molecule reacts.
🎯 By the end, you'll be able to
  • Identify the four hydrocarbon families: alkanes, alkenes, alkynes, aromatics
  • Use the general formula CnH2n+2 for alkanes and predict hydrogen counts
  • Explain saturated vs unsaturated in terms of C–C bonding
  • Recognise the benzene ring as the basis of aromatic hydrocarbons
📎 Helpful to know first

Why carbon is the star

Carbon has four valence electrons, so it forms four covalent bonds. That single fact makes it endlessly versatile: carbon atoms bond to each other in straight chains, branched chains and rings, and fill their remaining bonds with hydrogen. Compounds made of only carbon and hydrogen are called hydrocarbons.

We sort them into four families by the kind of carbon–carbon bonds they contain. Get the families straight and naming, reactions and properties all follow.

🔑 Saturated vs unsaturated
A hydrocarbon is saturated when every carbon–carbon bond is a single bond — the carbons hold as many hydrogens as possible. It is unsaturated when it contains a double or triple C–C bond, which means fewer hydrogens. "Saturated" literally means "filled up with hydrogen."

Alkanes: all single bonds

Alkanes have only single C–C bonds, so they are saturated. They follow the general formula CnH2n+2: methane (₁), ethane (₂), propane (₃), butane (₄), and so on. They are the backbone of natural gas and gasoline, and they are relatively unreactive — their main reaction is burning (combustion).

\[ \ce{CH4}\quad\ce{CH3CH3}\quad\ce{CH3CH2CH3} \]
Methane, ethane and propane — the first three alkanes, each carbon surrounded by single bonds.
✨ The formula is a counting shortcut
For any straight- or branched-chain alkane, hydrogens = 2 × (carbons) + 2. Hexane has 6 carbons, so 2(6) + 2 = 14 hydrogens: C6H14. Every double bond or ring you add removes two hydrogens from that maximum.

Alkenes and alkynes: the unsaturated pair

Alkenes contain at least one carbon–carbon double bond (C=C) and follow CnH2n. Ethene (C2H4) is the simplest and is produced by plants to ripen fruit. Alkynes contain a carbon–carbon triple bond (C≡C) and follow CnH2n-2; ethyne (acetylene) burns hot enough to weld metal.

The double and triple bonds are reactive sites — they readily undergo addition reactions, where atoms add across the multiple bond. That reactivity is exactly what saturated alkanes lack.

\[ \ce{CH2=CH2}\qquad\ce{CH#CH} \]
Ethene (a C=C double bond) and ethyne (a C≡C triple bond) — the simplest alkene and alkyne.

Aromatics: the benzene ring

The fourth family is aromatic hydrocarbons, built around the benzene ring — six carbons in a ring (C6H6) with the bonding electrons spread evenly around it. That shared "delocalised" ring is unusually stable, so benzene does not behave like a simple alkene despite being unsaturated. Aromatic rings appear throughout dyes, plastics and medicines.

⚠️ A structural formula is not the real shape
When we draw –CH2–CH2– in a straight line, that is a flat shorthand. The real molecule is three-dimensional: each single-bonded carbon is tetrahedral, so an alkane chain actually zig-zags in space. The paper drawing shows which atoms are connected, not the true 3D geometry.
📝 Worked example: Pentane is a straight-chain alkane with 5 carbons. Write its molecular formula and say whether it is saturated.
  1. Alkanes follow CnH2n+2, and n = 5 carbons.
  2. Hydrogens = 2(5) + 2 = 12.
  3. So the formula is C5H12.
  4. Every C–C bond is a single bond, so pentane is saturated (no double/triple bonds).
✓ C₅H₁₂, a saturated alkane.
✏️ Practice: An alkane has 8 carbon atoms (octane). Using CnH2n+2, how many hydrogen atoms does it have?
H atoms
Solution
  1. Alkane hydrogens = 2n + 2, with n = 8.
  2. = 2(8) + 2 = 16 + 2.
  3. = 18 hydrogen atoms, so octane is C8H18.

Check your understanding

1. Which hydrocarbon is unsaturated?
Ethene has a C=C double bond, so it is unsaturated. The alkanes ethane, propane and hexane have only single C–C bonds and are saturated.
2. What does it mean for a hydrocarbon to be 'saturated'?
Saturated means the carbons hold as many hydrogens as possible — only single C–C bonds. A double or triple bond makes it unsaturated.
3. A drawing shows butane as a straight horizontal line of 4 carbons. What does this tell you?
Structural formulas are 2D shorthand for connectivity. Each single-bonded carbon is tetrahedral, so the real chain is three-dimensional and bent, not flat.
🔎 Related lessons
Next: Functional Groups IUPAC Nomenclature Reaction Types & Mechanisms
✅ Key takeaways
  • Hydrocarbons contain only carbon and hydrogen; carbon's four bonds let it chain and ring.
  • Alkanes (CnH2n+2) are saturated — only single C–C bonds, relatively unreactive.
  • Alkenes (C=C) and alkynes (C≡C) are unsaturated and react by addition at the multiple bond.
  • Aromatics are built on the stable benzene ring (C₆H₆).
  • Structural formulas show connectivity, not the true 3D shape of the molecule.
➡️ Pure hydrocarbons are only the skeleton. Attach an oxygen, a nitrogen or a hydroxyl group and the molecule gains a whole new personality. Those reactive attachments — functional groups — are next.
Want to test yourself on this? Try the Chemistry practice test →